The present invention relates generally to combined turbine and generator units for hydroelectric plants deriving electrical energy from the subsurface water currents. It is especially useful in medium and small head plants for use in remote areas where regular electrical grid is not available. More specifically, the invention relates to hermetically sealed generators with self-adjusting axial gap.
Small underwater generators are generally known in the prior art. They typically include a turbine portion rotating a shaft and a generator portion for transferring into electricity of the kinetic energy of magnetic coils and alike of the rotor passing by the stationary coils of the stator. For underwater application, this general approach suffers from two problems: first, the shaft needs to be sealed, and second, a brush contact system for energizing the rotating coils has to be provided. The problem of longevity arises in submerged generators when the rotary shaft seal wears out and water gains access to the inside of the generator unit. Another problem stems from the wear of the electrical contact brushes, which require periodic replacement necessitating removal of the unit from the water.
Sealed underwater generators have been proposed in the prior art. One example of such is shown in the U.S. Pat. No. 3,209,156 by Struble, which is incorporated herein in its entirety. It shows a sealed stator containing a permanently attached shaft. An impeller is mounted on the shaft and contains a series of permanent magnets. Underwater current causes the impeller to rotate and therefore to move the permanent magnets next to the electrical coils of the stator of the unit whereby electricity is generated. Effectiveness of the generator directly depends on the optimal gap between the permanent magnets and the coils of the stator. This design incorporates a bearing system to support the rotation of the impeller, which is typical for these devices. The longevity of the device is limited due to the wear of the bearings causing the axial shift in the position of the impeller and therefore a decline in generating capacity of the device.
Guimbal shows another example of the permanent magnet underwater generator in the U.S. Pat. No. 2,634,375. The generator unit is coupled directly with the turbine unit, which obviates the need for a complex gearbox or another type of a transmission. This design relies on a source of pressurized oil to fill in the space of the generator to lubricate it and to transfer away the heat that is generated by the unit. Such a provision may not be practical in remote areas where the generator of this type may be used most advantageously.
A further yet example of the small generator is shown in the U.S. Pat. No. 2,127,847 by Schulte. A nautical instrument measures a speed of the boat to which it is mounted. A generator has a small impeller equipped with a series of permanent magnets, which is passively turned on by the moving boat. The faster the boat is the more electricity is generated therefore the speed may be estimated based on the generator output. There is no provision again here for any self-adjustment of the axial gap due to the wear of the axial bearing system, which ultimately may cause a decline in the instrument accuracy.
Finally, a self-adjusting axial gap design of the motor/generator is shown by Miller in the U.S. Pat. No. 5,627,419. A spring-loaded flywheel is shown to contain a series of permanent magnets rotating next to a stator having a series of electromagnetic coils. When engaged with the stator, the gap is maintained small due to the tapered design of the flywheel and the stator and the axial forces generated by the torque of the device. When the flywheel is allowed to rotate freely, it shifts axially away from the stator so that the electromagnetic drag is minimized. This design has two distinct main positions of the flywheel and does not allow for infinite position adjustment in response to the wear of the axial bearing.
The need exists therefore for a small sealed underwater generator having a simple and reliable design for use in remote areas with high reliability and longevity. The need also exists for an underwater generator capable of working in small rivers and alike. In such conditions, water flow may be heavily contaminated with floating particles such as sand. Sand filled water may cause rapid wear of the components of the traditional underwater generators resulting in an increase of the axial gap of the turbine and a decline in energy generating capacity.
Accordingly, it is an object of the present invention to overcome these and other drawbacks of the prior art by providing a novel sealed directly coupled underwater generator with self-adjusting axial gap to maintain the high electrical energy generating capacity despite the wear of its components.
It is another object of the present invention to provide a sealed generator with long operational life and extended periods between scheduled maintenance and repairs.
It is a further object of the present invention to provide an underwater generator capable of functioning in water currents heavily contaminated with floating particles such as sand.
It is a further yet objective of the present invention to provide an underwater generator with a capacity to maintain the optimal axial gap over a wide range of operating parameters including changes in the upcoming water current.
These objectives of the invention are achieved by providing a generator having its rotor directly coupled to its turbine. The three-phase synchronous generator includes a series of permanent magnets sealingly incorporated into its rotor. The rotor is rotated on a shaft by the incoming water flow in such a way that the permanent magnets move about and in close vicinity to the alternating electromagnetic coils of the stator. The objectives of the present invention are realized by providing a rotor with the ability for axial shift along the shaft. The rotor is spring loaded at the end of the shaft to force it towards the housing in the direction opposite the flow direction of the incoming water current.
The rotor is provided with inside channels and cavities, which are organized in such a way that the axial gap between the rotor and the stator is maintained in the optimal range. The gap is defined by a balance between the force of the spring and the axial pressure of the water current regardless of the wear status of the bearings. This is achieved for the most part by providing a constant flow auxiliary pump driven by the turning rotor. The pump is infusing a constant amount of water along the vane channels in the rotor and into the gap on the periphery between the rotor and the stator. That flow develops an axial hydrodynamic force against the rotor. A combination of that force in addition to an electromagnetic force and the force of the spring determines the final axial gap between the rotor and the stator. Wear of the rotor from the incoming flow of suspended particles of sand and alike result in increase of the gap but that in turn leads to reduction on flow resistance from the auxiliary pump and subsequent drop in the hydrodynamic forces component. A balance of forces is therefore disturbed and the rotor is shifted closer to the stator by the spring. This automatic action increases the flow resistance, raises the hydrodynamic forces and brings the axial gap back into the acceptable range. The flow channels are made wide enough to allow for flowing water to carry through any amounts of particles including sand without trapping thereof inside the unit.